Aerosol particle depolarization ratio at 1565 nm measured with a Halo Doppler lidar

Abstract

Abstract. The depolarization ratio is a valuable parameter for lidar-based aerosol categorization. Usually, the aerosol particle depolarization ratio is determined at relatively short wavelengths of 355 nm and/or 532 nm, but some multi-wavelength studies including longer wavelengths indicate strong spectral dependency. Here, we investigate the capabilities of Halo Photonics StreamLine Doppler lidars to retrieve the particle linear depolarization ratio at the 1565 nm wavelength. We utilize collocated measurements with another lidar system, PollyXT at Limassol, Cyprus, and at Kuopio, Finland, to compare the depolarization ratio observed by the two systems. For mineral-dust-dominated cases we find typically a slightly lower depolarization ratio at 1565 nm than at 355 and 532 nm. However, for dust mixed with other aerosol we find a higher depolarization ratio at 1565 nm. For polluted marine aerosol we find a marginally lower depolarization ratio at 1565 nm compared to 355 and 532 nm. For mixed spruce and birch pollen we find a slightly higher depolarization ratio at 1565 nm compared to 532 nm. Overall, we conclude that Halo Doppler lidars can provide a particle linear depolarization ratio at the 1565 nm wavelength at least in the lowest 2–3 km above ground.

Lidar depolarization ratio of atmospheric pollen at multiple wavelengths

“…nent (BÅE) at 355/532 nm of −0.64 ± 0.45. Cases of negative Ångström exponents have previously been observed for dust particles (Bohlmann et al, 2018;Rittmeister et al, 2017;Tsekeri et al, 2017;Veselovskii et al, 2016) and are likely caused by specific chemical compositions causing a spectral variation of the imaginary part of the refractive index (Veselovskii et al, 2016). The BÅE at 532/1064 nm, however, is positive (0.97 ± 0.41) and comparable with values of dust smoke mixtures (Kanitz et al, 2014;Tesche et al, 2011).…”

“…For cases with a high amount of large, non-spherical spruce pollen, e.g., with > 15 % share of the total pollen number concentration, the BÅE at 355/532 nm is negative. Previously, negative Ångström exponents have only been reported for dust close to the dust source region where very large dust particles are present (Hofer et al, 2020;Rittmeister et al, 2017;Veselovskii et al, 2016). This feature, however, could also be characteristic of aerosol conditions with the presence of very large pollen grains.…”

Characterization of atmospheric pollen with active remote sensing

“…Furthermore, the new system measures the depolarization ratio also at 355 nm, while depolarization measurements were limited to 532 nm in the previous campaign. To further improve the investigation of pollen depolarization properties, we assessed the depolarization measurements of a co-located Halo Doppler lidar, which provides information at 1565 nm (Vakkari et al, 2021). The measurement of the depolarization ratio at multiple wavelengths allows us to investigate its wavelength dependence.…”

“…This lidar operates at a wavelength of 1565 nm with a pulse repetition rate of 15 kHz and a pulse energy of 20 μJ. The minimum range of the instrument is 90 m and the range resolution is 30 m; other operating specifications can be found in Vakkari et al (2021). This Halo Doppler lidar is equipped with a cross-polar receiver channel, which enables consecutive measurement of co-and cross-polar signals in vertically pointing mode and thus the retrieval of the depolarization ratio at 1565 nm.…”

“…Therefore, depolarization measurements can only be calibrated using the depolarization ratio at liquid cloud base. As liquid cloud droplets do not polarize the backscattered light, non-zero depolarization ratios at cloud base indicate incomplete extinction, called bleed-through, in the internal polarizer (Vakkari et al, 2021). The bleed-through in the Halo internal polarizer was estimated from liquid cloud base observed at 1200-1400 m a.g.l.…”